1. Field of the Invention
The present invention concerns a measuring device for a flame. The invention also concerns a regulating device for a burner having the measuring device.
2. Description of the Prior Art
DE 19632 983 A1 discloses a measuring device for a flame and an associated regulating device in a gas burner, wherein a lambda reference value for low emissions is set by means of an ionization electrode. By means of a comparator, the analog signal is digitised for further processing. The signal produced by the comparator however involves only a slight signal variation and a small signal-noise spacing at the on-off threshold if the signal is also to be used for flame monitoring purposes.
The object of the present invention is to provide a measuring device for a flame, which permits more accurate and improved signal evaluation.
In accordance with the invention, there is provided a measuring device for a flame, in particular for use in a regulating device for a burner, comprising an ionization electrode which is positionable in the flame region of the burner and to which an ac voltage is applied whereby a dc voltage component is superimposed in dependence on the ionization current, wherein the ac voltage component which is influenced by the flame resistance can be separated from the dc voltage component by way of first means and the separated ac voltage can be compared to the separated-off dc voltage component by way of second means in order to produce a pulse width-modulated signal.
An important concept of the invention is that the alternating component which is influenced by the flame signal can be separated from the dc voltage component by way of first means and the separated alternating component can be compared to the separated-off dc voltage component in order to produce a pulse width-modulated signal.
Fluctuations in the amplitude in the supply voltage are compensated by the comparison of the alternating component to the direct component as both components change in terms of amplitude in the same relationship. In contrast changes in the flame, for example due to changes in the air ratio, influence the two components unequally.
Further advantages are the signal variation which can be adjusted in a wide range, the high level of sensitivity and the large signal-noise spacing as to whether the flame is on or off, and the fact that the analog signal is highly accurate and reproducible.
Further advantageous aspects of the invention are set forth in the dependent claims.
Thus, signal transmission by way of an optocoupler is possible, in which case both items of information, flame on and off and PWM-signal, can be transmitted by way of just one optocoupler. The ionization electrode can be designed to be shock-proof by virtue of the installation of contact shock-protection resistors.